The present invention relates to a spark plug for an internal combustion engine and a method for manufacturing the same, and more particularly to a spark plug provided with a noble metal tip on the center electrode top end opposing the ground electrode and a method for manufacturing the same having such a center electrode.
Spark plugs used to ignite a combustible mixture supplied to internal combustion engines are subjected, during long use, to electrode wear which degrades the spark discharge characteristics of the spark plug. It has been suggested that the spark plug be provided with a noble metal such as platinum on the top end of the center electrode where the spark discharge is generated to thereby enhance the durability and hence maintain the desired spark discharge characteristics.
One type of spark plug suggested in Japanese Utility Model publication No. 53-38046 and Japanese Patent Laid-open No. 51-66945, for example, is provided with a noble metal tip plugged up in a recess drilled axially in the center electrode top end opposing the ground electrode. This spark plug requires a drilling process to form the recess and requires a considerable quantity of noble metal tip to fill the recess to the extent that the noble metal tip is free of attrition or the like, thus resulting in impracticality for commercial production with respect to manufacturing cost.
The other type of spark plug suggested in Japanese Utility Model Laid-open No. 54-92227, for example, is provided with the noble metal coating over the center electrode top end. The bonding between the center electrode and the coating is not sufficient to prevent attrition of the coated noble metal caused during long use by the heat deterioration of the noble metal and the oxidization of the electrode material, thus also resulting in impracticality for commercial production with respect to product durability.